Electronic amplifiers are fundamental building blocks used in virtually every field of electronics, and over the years a great number of amplifier designs have evolved for specific applications. The designer of an amplifier or an electronic system using amplifiers must make choices among various performance and cost parameters in trying to achieve the best amplifier for a given application. Such variables as input and output impedance, voltage gain, current gain, forward transconductance, forward transimpedance, frequency response, distortion, bias requirements, stability, and the like, must be considered and often balanced one against the other for a given application. For many applications, integrated circuit operational amplifiers are selected with the intent that external connections for input and feedback will give the required operating characteristics, but this is not always the case. All real amplifier designs including operational amplifiers fall short of ideal characteristics to some extent, and the nature of the particular shortcomings will determine how well a given amplifier or operational amplifier will perform in a given application. Moreover, at the integrated circuit design level, the designer is still faced with the task of designing and laying out device configurations to achieve the desired amplifier characteristics. In many cases, shortcomings in certain amplifier performance parameters have been overcome by more complex designs having higher gain which permits greater amounts of negative feedback to linearize and stabilize the amplifier. However, such approach can lead to problems in transient response and high frequency response.
In many applications a simple but rugged amplifier is needed having low distortion characteristics for driving some downstream device or circuit in response to an input current. Numerous examples occur in the fields of audio, video, communications, computers and control systems. An ideal transimpedance amplifier transforms an input current to an output voltage by a given ratio, and would ideally have zero input and output impedance, infinite frequency response and slew rate, zero distortion, zero input bias currents, unlimited available output current, and transimpedance determined only by feedback impedance. The ideal transimpedance amplifier would also have infinite voltage gain, infinite current gain, and infinite forward transconductance.
A transimpedance amplifier is usually configured as an amplifier having a transfer function H(s) within a feedback network Z(s). If the characteristics of the amplifier portion are ideal (infinite input impedance, zero output impedance, infinite voltage and current gain, infinite forward transconductance, infinite forward transimpedance, infinite frequency response, infinite slew rate, zero distortion, zero input bias currents, and unlimited available output current), then the transconductance amplifier constructed will be ideal, with the transimpedance determined by the value of the feedback impedance Z(s).
One simple type of transimpedance amplifier uses a single stage bipolar transistor amplifier connected in a common emitter configuration. The chief advantages of the amplifier portion in this configuration are its low cost, high transconductance and high voltage gain. However, the current gain and forward transimpedance are relatively low. This type of circuit is often improved by providing an input emitter follower stage followed by the common emitter stage, to provide improvements in current gain and forward transimpedance. Another alternative uses a pair of transistors connected as a Darlington pair, to provide much the same advantages. Another commonly used amplifier design is a differential input amplifier stage. This can provide about twice the current gain and twice the forward transimpedance of a simple common emitter bipolar stage. All of these circuits can be further improved by adding additional stages to increase the amplification, but at the disadvantage of added cost, complexity, and in some cases, distortion.
In the above-described prior art circuits, the current gain and forward transimpedance may still be inadequate for many purposes. In addition, an unsatisfactory trade-off must be made between low input bias currents or fast slew rates, but not both simultaneously.